1. Field of the Invention
The present invention relates to, for example, an actinic energy curable urethane resin suitably used as adhesives, inks, and coating materials. Particularly, the present invention relates to an actinic energy curable urethane resin suitably used as an actinic energy ray curable adhesive for forming a back protective sheet for a solar cell.
Further, the present invention relates to an actinic energy ray curable adhesive that makes it possible to produce a back protective sheet for a solar cell excellent in adhesion between sheet members and moist-heat resistance with high yields and high productivity at low cost.
2. Description of the Related Art
Recently, awareness of environmental concerns has been increasing, and solar cells attract attention as they are clean energy sources and do not cause environmental pollution. Since the solar cells allow solar energy, which is a substantially inexhaustible energy resource, to be used as electric energy, they have been intensively studied and already put into practical use.
In order to protect solar cell elements, a solar cell is provided with a back protective sheet on the surface opposite to the surface from which sunlight enters into the cell. The back protective sheet for a solar cell is required to deliver performance such as weather resistance, water vapor permeability, electrical insulation, mechanical characteristics or mounting workability.
As the back protective sheet for a solar cell, a sheet formed by stacking several sheet members is usually used. In the stacking of these sheet members, generally used is a polyurethane adhesive that contains a hydroxyl group-containing resin as a base resin and an isocyanate compound as a curing agent as described in Jpn. Pat. Appln. KOKAI Publication No. 2007-320218 and Jpn. Pat. Appln. KOKAI Publication No. 2007-253463.
When the back protective sheet for a solar cell is mass-produced, a plurality of sheet members are stacked to form a long laminate, and then the long laminate is wound into a roll. However, the curing reaction of the polyurethane-based adhesive is slow, and thus the stacked sheet members of the wound laminate are likely to slip with respect to each other. Thus, a process using a polyurethane adhesive has problems such as high rejection rate and low yield.
In order to allow the polyurethane-based adhesive to be sufficiently cured, the above-described laminate needs to be subjected to aging in a warehouse maintained at a high temperature over several days. Therefore, the process using the polyurethane-based adhesive has problems such as low productivity and requirement of electric costs for maintaining the temperature in the warehouse.
Further, the isocyanate compound, which is a curing agent, reacts with not only the hydroxyl group-containing resin, which is a base resin, but also water in the air. A product of addition reaction of the isocyanate compound with water causes decarboxylation, and thus air bubbles are generated in the adhesive layer after stacking the sheet member. Accordingly, the process using the polyurethane-based adhesive has problems such as appearance defects and delamination.
Jpn. Pat. Appln. KOKAI Publication No. 2008-127475 discloses an actinic energy ray curable resin adhesive prepared by reacting polycarbonatediol, a bifunctional epoxy(meth)acrylate having two hydroxyl groups and two ethylenically unsaturated groups in a molecule, and polyisocyanate with one another and mixing an unsaturated group-containing urethane resin thus obtained with a photopolymerization initiator. However, when this is used as an adhesive for a back protective sheet of a solar cell, the adhesion between the sheet members and moist-heat resistance are not sufficient.